⚙️ ⚙️ Phase 5 — Framework Internals (AMS, WMS, PMS)

Duration: 4–6 weeks | Level: Advanced

The Android Framework is the bridge between your app and the OS. Understanding it makes you a 10x Android engineer.

🏛️ The Big Three System Services

All three live in frameworks/base/services/core/java/com/android/server/

⏳ ActivityManagerService (AMS)

AMS is the most complex service in Android. It manages:

• Activity stack management (back stack, tasks)
• Starting / killing app processes
• Broadcasting (sendBroadcast)
• Starting Services
• App lifecycle (foreground/background)
• Process memory pressure (LRU cache of processes)

startActivity() deep dive

Activity.startActivity(intent)
  ↓
Instrumentation.execStartActivity()
  ↓
ActivityTaskManager.getService().startActivity()   # Binder call to AMS
  ↓
ActivityTaskManagerService.startActivity()
  ↓
ActivityStarter.execute()
  ↓
RootWindowContainer.resumeFocusedTasksTopActivities()
  ↓
ActivityRecord.makeActiveIfNeeded() → realStartActivityLocked()
  ↓
ClientTransaction sent to app process via Binder
  ↓
ActivityThread handles LaunchActivityItem
  ↓
Instrumentation.callActivityOnCreate() → Activity.onCreate()

Key AMS files

frameworks/base/services/core/java/com/android/server/am/
├── ActivityManagerService.java      # Main service
├── ProcessRecord.java               # Represents one app process
├── BroadcastQueue.java              # Broadcast dispatch
├── ActiveServices.java             # Service lifecycle
└── OomAdjuster.java                # Process kill decisions
 
frameworks/base/services/core/java/com/android/server/wm/
├── ActivityTaskManagerService.java  # Activity-specific (split from AMS in Android 10)
├── ActivityRecord.java              # Represents one Activity instance
├── Task.java                        # Back stack task
└── ActivityStarter.java             # Start activity logic

🪟 WindowManagerService (WMS)

WMS manages every window on screen:

• Assigns Z-order (layers) to windows
• Handles input event routing to correct window
• Coordinates with SurfaceFlinger (native compositor) for rendering
• Manages window animations and transitions

Window hierarchy

Display
  └─ RootDisplayArea
       ├─ StatusBar window
       ├─ NavigationBar window
       └─ AppWindowToken (per app)
            └─ WindowState (per window surface)

SurfaceFlinger relationship

• WMS manages which windows exist and their properties
• SurfaceFlinger (native, C++) does the actual compositing (merging layers into final pixels)
• Apps draw into their Surface buffers; SurfaceFlinger reads them

Key files:

frameworks/base/services/core/java/com/android/server/wm/WindowManagerService.java
frameworks/native/services/surfaceflinger/SurfaceFlinger.cpp

📦 PackageManagerService (PMS)

PMS handles everything related to installed apps:

• Parsing AndroidManifest.xml of every installed APK
• Storing component info (Activities, Services, Receivers, Providers)
• Resolving intents to components
• Enforcing permissions
• APK install/uninstall pipeline

APK install flow

User taps "Install"
  ↓
PackageInstaller UI → PackageInstallerSession
  ↓
PMS.installPackage()
  ↓
PackageParser.parsePackage()   # Parses AndroidManifest.xml
  ↓
dex2oat (ART compilation of .dex files)
  ↓
Copy to /data/app/<package>/
  ↓
Update package database (packages.xml)
  ↓
Broadcast ACTION_PACKAGE_ADDED

🔥 Other Important Framework Components

InputManagerService

• Reads raw input events from kernel (/dev/input/eventX)
• Routes touch/key events to the focused window via WMS
• Key file: frameworks/base/services/core/java/com/android/server/input/InputManagerService.java

ContentProvider & ContentResolver

• IPC mechanism for structured data sharing between apps
• Uses Binder under the hood
• SQLite + URI-based query interface

BroadcastReceiver dispatch

sendBroadcast(intent)
  ↓
AMS.broadcastIntent()
  ↓
BroadcastQueue.enqueueOrderedBroadcastLocked()
  ↓
BroadcastQueue.processNextBroadcast()
  ↓
App process → ActivityThread.handleReceiver()
  ↓
Receiver.onReceive()

🛠️ Hands-On Tasks

1. Add a log to ActivityManagerService.java in startActivity() and trace a real launch.
2. Add a log to PackageManagerService.java in scanPackageLI() — watch it during adb install.
3. Read OomAdjuster.java — understand how the system decides which process to kill.
4. Use adb shell dumpsys activity to inspect live AMS state.
5. Use adb shell dumpsys window to inspect live WMS window state.
6. Use adb shell dumpsys package <your.package> to see parsed manifest info.

✅ Phase 5 Checklist

• Can explain the roles of AMS, WMS, and PMS
• Traced startActivity() from app to Activity.onCreate()
• Read ActivityRecord.java and Task.java
• Understand window hierarchy in WMS
• Know SurfaceFlinger's role vs WMS
• Traced APK install flow through PMS
• Understand OomAdjuster — how processes are killed
• Understand BroadcastQueue dispatch
• Used dumpsys activity, dumpsys window, dumpsys package
• Modified a system service and built incrementally